The long-term objective of this project is the elucidation of the mechanisms that guide neurons through the process of selecting particular synaptic targets and control the features of these connections. Our previous studies of the visual system of Daphnia magna have centered on the selection process. Our specific aim for the period of this proposal is to examine, in quantitative detail, the structural features of synaptic connections between specific neurons in order to answer several important questions: To what degree are the fine structural features of the synaptic connections between identified neurons the same among animals with the same genetic background and raised under identical conditions? How are these features affected by changing environmental conditions? How do they change with the age of the animal? Does variation or constancy of synaptic features correlate with variation or constancy of behavior? Daphnia are simple enough to permit the detailed quantitative studies needed for answering these questions, yet complex enough to have sophisticated optomotor behaviors. Our approach is multidisciplinary, employing anatomical, behavioral and physiological techniques. The quantitative anatomical measurements will be carried out using the techniques for computer-aided reconstruction of nerve cells from serial electron micrographs which we have developed. Properties we will measure include number of synaptic sites, their area and distribution. In the behavioral studies we will use video recording to measure eye movements produced by various patterns of visual stimulation. The physiological studies will include recording from photoreceptors and interneurons involved in the optomotor behaviors. We expect to obtain answers to basic questions about the structure, function and development of synaptic connections which will contribute significantly to the understanding of how abnormalities arise in the structure and function of the nervous system.